Add support for the not equal operator

[lognorman20]

Adds the != operator for variables in Noname, closes #113.

Summary of changes:

• Implemented is_zero_cell to check if a given cell is equal to zero. This function closely follows the Gnark implementation. I'm imagining this can be further simplified to reduce the total number of constraints necessary -- perhaps taking into account the Circom implementation.
• Implement not_equal to check if two given cells are equal. The check is done by calculating the difference between the two and checking if it's zero
• Update the lexer, parser, writer, and type checker to recognize !=
• Add example of using != in examples/not_equal.no
• Add a test for the != operator

[mimoo]

code looks good to me, but as pointed out do you want to try first in another PR if the solution using equal_cell leads to less constraints?

[lognorman20]

yes i can try to reduce constraints in another pr

[lognorman20]

@mimoo addressed in #128!

[mimoo]

maybe is_equal should use is_zero(a-b) now?

anyway thanks so much!